In the processing of heavy crude petroleums, oil sands, and similar raw materials, a common approach to the problem of their low hydrogen-to-carbon ratio is to diminish their carbon content by various types of coking processes. Generally in these processes a large amount of predominantly carbonaceous material (coke) is formed, whose high heating value makes it useful for process heating in the extraction plants. In the Flexicoking (Trade Mark) fluid coking process in which most of the coke is gasified after passing through the fluid coker, smaller amounts of coke are produced per unit of crude petroleum or other feed and consequently the ash and vanadium content of the coke from such process is higher than that of coke from other coking processes. Fly ash, a by-product of conventional combustion of petroleum coke, is not entirely free of carbonaceous materials and generally contains about 50% ash and 50% coke granules, depending upon the particular combustion process and the feedstock. The aforementioned ash portion of the coke and fly ash materials contains substantially all of the metallic minerals from the original heavy crudes, including the vanadium which occurs in heavy crudes in the form of porphyrins. The coke may also contain nickel compounds, and less valuable materials such as alumino-silicates and other inorganic compounds that may be carried over with the bitumen in the separation processes that are commonly employed in extracting bitumen from sand in mineable oil sands deposits. Some of the metallic constituents, particularly the vanadium and the nickel, have significant commercial value. Furthermore, if the metallic materials are deposited as tailings at plant sites or elsewhere, they are subject to leaching by rain and can cause contamination of ground water supplies, a potentially significant environmental hazard. It is therefore desirable to separate the commercially valuable materials and to render the residue as harmless as possible.
A known method for recovering metals contained in the residue from the distillation of heavy hydrocarbons is direct leaching of by-product fly ash. Acidic and alkaline solutions as well as water have been utilized as the leach solvent, but generally these methods exhibit poor yield of vanadium values and do not adequately separate the vanadium from any contaminants that may be present. As the acidity of the leach solvent increases, the yield of recovered metal usually increases, but the improvement is usually obtained at the cost of greater carryover of impurities, to the point where if 2-Normal sulphuric acid is employed, many of the components in the fly ash appear in the overflow solution and there remains the problem of separating a complex mixture, except that it is in the liquid (solution) phase.
Stemerowicz et al (Canadian Institute of Mining and Metallurgy Bulletin, April 1976, pp. 102-108) disclosed separation of a mineral-rich portion from a fly ash by-product of Suncor, Inc. at Fort McMurray, Alberta by flotation or alternatively, by combustive roasting of the coke to leave a mineral-rich portion as ash, and smelting of the mineral-rich fraction to obtain ferro-vanadium or metal alloys containing the vanadium along with other metals such as iron and silicon, in the elemental state. Losses of vanadium to the slag were 14% in the first stage and 20% to 58% in the second stage, and the product required further treatment to purify the vanadium.
In Vesely's patent, U.S. Pat. No. 3,522,001, a process was disclosed comprising mixing alkali metal halide with coke and burning the mixture at temperatures sufficiently high to fuse the metals and the salt, scrubbing the fused residue with a weak solution of sulphurous acid, forming a sulphurous acid solution of nickel chloride and slurry containing alkali metal vanadate, separating and recovering vanadium pentoxide from the slurry and performing three other steps to recover nickel oxide. No percentage yield of vanadium or purity of the product was disclosed in that patent.
As early as 1906, Handy (U.S. Pat. No. 831,280) disclosed the roasting of ores with any of several alkali salts, followed by water leaching. Although no temperature conditions were taught in the patent, it is clear that the process was operated well above the point of fusion of the salts because it was noted that alumina and silica were solubilized, an effect that occurs only substantially above the fusion point of the salt and in the presence of a large excess of salt.
Indeed, all of the known prior art processes appear to use temperatures high enough to fuse the roasting agent, and generally, increased temperatures are said to cause increased yields of metal values. Contrary to the teaching of the prior art disclosures it has been discovered that greatly increased recovery of highly purified vanadium can be obtained by operating at temperatures below the fusion point of the salt present during a heating stage in the novel process claimed herein.
The Handy patent asserted a need to remove sulphur prior to fusion of the ore with the salt. However, this approach unnecessarily consumes reagent; it has been found that high sulphur content of the raw material, measured as high as 12%, does not hinder recovery of the vanadium in the present invention. Furthermore, the presence of calcium is stated in the art (Lundquist, U.S. Pat. No. 2,640,754) to impede vanadium recovery from certain ores; however, it has been found that, in the process disclosed herein and without prior treatment with concentrated sulphuric acid, a high recovery can be obtained regardless of calcium (measured as Ca) in the raw material in a proportion up to at least 4.6%.
Prior art processes in general exhibit poor yields, high chemical consumption, and/or low specificity when applied to cokes and ashes derived from oil sands bitumen. Indeed, in a paper entitled "Potential for Recovering Vanadium from Athabasca Tar Sands", presented at the 26th Canadian Chemical Engineering Conference, Toronto, Oct. 3-6, 1976, L. A. Walker, R. W. Luhning and K. Rashid concluded, after tests which consumed excessive amounts of chemicals while yielding a maximum of 35% vanadium recovery, that "There is at the moment no economically feasible commercial recovery method."
In the following disclosure and claims, "alkali metal vanadate" comprehends both metavanadate, MVO.sub.3, and pyrovanadate, M.sub.4 V.sub.2 O.sub.7. All parts, proportions and percentages in the disclosure and claims are by weight unless specifically indicated otherwise.